Method of and apparatus for the drying and heating of gravel and crushed stone



United States Patent 3,362,078 METHOD OF AND APPARATUS FOR THE DRYING AND HEATING 0F GRAVEL AND CRUSHED STONE Erich Kamper, Mulhcim (Ruhr), and Heinrich Giilden,

Essen, Germany, assignors to Firma Essener Teerschotter G.m.b.H., a corporation of Germany Filed Apr. 25, 1966, Ser. No. 545,160 Claims priority, application Germany, Apr. 23, 1965, E 29,169 5 Claims. (Cl. 34-9) ABSTRACT OF THE DISCLOSURE Method and apparatus for the drying of gravel and crushed stone without loss of fine-particle fraction and without the formation of dust wherein the crushed stone (including the fine fraction) is floated in a thin layer upon the surface of a lead-tin melt (e.g., 67% tin, 33% lead) having a melting point less than 200 C. but above 100 C. while heating this bath from below, the stone being removed from the surface of the melt without substantial displacement of the bath after the stone has dried.

Our present invention relates to a method of and apparatus for the drying of granular minerals such as crushed stone, gravel and the like and, especially, comminuted minerals adapted to be combined with a bituminous binder in tar road-paving compositions and the like.

It is common practice to heat-treat comminuted minerals such as rock shapes and gravel to dry them prior to the application of tar or bituminous binder to the granular minerals in order to facilitate adhesion of the asphaltic mass. Heretofore, rotary-drum dryers and calcining-type rotary kilns have primarily'been employed forthis purpose. Such devices had inclined furnace drums provided with combustion burners at the lower end and means for supplying the crushed stone or other granular minerals to the upper end of the drum; the hot gasses passed at relatively high velocities through the drum interior in countercurrent to the descending particle stream. Because of the high gas velocities, the chimneys or other exhaust systems connected to the drum at its upper end were required to be equipped with dust-removing devices in order to prevent atmospheric pollution. Not only were such devices relatively expensive, but the entire system resulted in an entrainment of the smaller particles by the gas stream such that the smaller particles no longer were present in the product. This disadvantage is especially important when the aggregate is to be used in road-paving compositions and the like since it has been found that the best compositions are those which include a substantial proportion of the fine aggregate together with aggregate of comparatively large particle size.

It is, therefore, an important feature of the present invention to provide an improved method of drying mineral aggregates and granular masses whereby the fine particles are not stripped from the mass and in which the heating efficiency is substantially improved by comparison with conventional methods, the method being carried out with relatively low equipment cost and without pollution of the atmosphere.

Still another object of this invention is to provide an improved apparatus for the heating and drying of mineral granules adapted to obviate the aforementioned disadvantages of earlier systems. I

We have found that these objects and others which will become apparent hereinafter, can be attained by a unique system for the drying of granular mineral masses in which the mineral is deposited upon the surface of a heated melt with a specific gravity in excess of that of the mineral at a temperature substantially in excess of the boiling point of water, carried along the surface in a relatively thin layer upon this melt until the granular mass is dry, and thereafter separated fromthe melt. Advantageously, the melt is composed of one or more relatively low-melting metals which are maintained in a bath at a temperature above the boiling point of water while the mineral granules are conveyed substantially continuously along the surface of this bath relative to the melt or in entrainment therewith from an input side of the melt, at which the granules are deposited upon the surface, to an output side at which the granules are removed. The melt can be a lead-tin alloy having a melting point advantageously below 200 C. (e.g., 67% tin, 33% lead). The heating of the mineral is effected by direct contact with the metal bath upon which the granules float so that a high heattransfer efficiency is obtained. Moreover, relative displacement of the bath and the granules in accordance with a specific aspect of this invention greatly facilitates the uniformity of the heating.

According to a more specific feature of this invention, an apparatus for the heating of granular minerals in the manner set forth comprises an open-top pan or trough containing a metallic melt in the manner described, inlet or feed means at one side of the bath for depositing the granular minerals upon the surface thereof, means on another side of the bath for discharging the dried granules, and means for conveying the floating mineral granules along the surface of the bath between the feed means and the discharge means. As previously indicated, the granules can be transported along the bath surface by movement relatively thereto or by entrainment with the melt so that the transport or conveying means can include circulating means for establishing a current of the molten metal across the bath from the feed means to the discharge means. Such circulating means may have one or more pumps providing a molten-metal circulation path drawing the melt from the discharge side and returning it to the feed side. According to another feature of the invention, however, the melt remains substantially stationary while endless conveyor means is disposed above the surface and is provided with plates adapted to sweep just slightly above the surface and carry the granules toward the discharge side. In this case, the endless conveyor means is so constructed and arranged that there is substantially no movement of the molten mass. The rake plates of the conveyor means depend from one or more belts or chains disposed above the melt to a level just above the surface of the melt and suflicient to engage the granules floating upon this surface. The transport speed is so selected with respect to the temperature of the melt that a complete dnying is effected. It has been found, surprisingly, that under the conditions described there is substantially no adhesion of the molten mass to the solid phase namely, the mineral granules.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following descrpition, reference being made to the accompanying drawing in which:

FIG. 1 is a diagrammatic vertical cross-sectional view through a preferred type of apparatus for carrying out the present invention; and

FIG. 2 is a fragmentaary longitudinal cross-sectional view through a modified mineral-drying system.

In FIG. 1 of the drawing, we show a particularly advantageous embodiment of an apparatus for the drying of particular minerals with a minimum formation of dust which comprises a trough, pan or other receptacle 2 re ceiving a molten-metal bath 3 preferably of the lead-tin alloy mentioned above. The bath 3 surmounts a furnace chamber 1 formed by thermally insulating and preferably refractory walls 4 at one end of which is provided an array of gas burners represented at 5. It will be understood, of course, that any conventional combustion means (e.g., for atomized liquid fuels, powdered-solid fuels or the like) can be used. At the other end of the furnace chamber 1, remote from the burner 5, an outlet 4 is provided in connection with a chimney hood 7 whose duct 7 communicates with a stack or waste-heat recovery system. The combustion gas thus heats the bath by passing along the bottom of the receptacle. At the inlet side of the bath 3, we provide feed means represented by a chute adapted to deposit a layer of particulate minerals M upon the surface S of the bath. Since the bath has a specific gravity substantially in excess of the specific gravity of the rock, the mineral particles, granules or chips float upon the surface S and may be conveyed therealong in the direction of arrow D toward an outlet or discharge means 8 at the opposite longitudinal end of the bath. In this embodiment, the discharge means 8 is constituted by a ramp lying substantially at the level of surface S and adapted to receive the crushed-rock chips or gravel transported across the surface S. The transport means here includes an endless conveyor 9 carried by a pair of rollers 9a and 9b overlying the surface .S of the metal bath 3 so that a stretch 9' of the conveyor means is substantially parallel to the surface from the inlet means 6 to the discharge means 8. From the endless conveyor 9, there depends scraper plates of baflles 10 which terminate at a level L slightly above the surface S so that these plates can entrain the granules M without material movement of the molten metal to sweep the granules across the surface S of the bath and carry them onto the ramp 3. A motor 9c is provided to drive the conveyor 9 and is provided with speed control means 9d which can be adjusted in accordance with the moisture content of the mineral particles to set the conveyor speed such that complete drying is effected in a single pass of the minerals across the surface of the path 3. The direct contact of the particles with the liquid metal ensures a high rate of heat transfer to the latter and thus a rapid drying of the thin layer of mineral particles disposed upon the surface of the bath.

In the embodiment of FIG. 2, the trough 22 containing the bath 23 of molten metal is again disposed above a furnace chamber 21 but the transport of the mineral particles M from the inlet chute 26 to the outlet ramp 28 is effected by a stream of molten metal. For this purpose, a circulating pump 29c is provided in a circulationpipe system 29 whose inlet 29a is disposed under a perforated collecting plate 291) and whose outlet 29c is disposed substantially at the level of surface S of the bath at the opposite end thereof. It will be evident that the pumping action of the circulating means causes a surface current of molten metal (represented by arrow D) to convey the particles M across the surface S to the collecting plate 2% at which the liquid is separated from the mineral particles which proceed along the discharge ramp 28, the liquid being recirculated to the mouth 29c of the recirculating means.

The invention described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the appended claims.

We claim:

1. A method of drying particulate stone without loss of fine fractions and without development of dust, which comprises the steps of: heating a bath of molten lead-tin alloy having a melting point greater than the boiling point of water and less than 200 C. and a specific gravity greater than that of the stone; depositing the stone particles in a thin layer upon the surface of said bath at a temperature above the boiling point of water whereby said stone floats upon said bath; transporting said particles across said surface from an inlet side to a discharge side of the bath at a rate such that said particles remain upon said surface for a time sufficient to effect a substantially complete drying of the particles and thereafter removing said particles from said melt.

2. The method defined in claim 1 wherein said particles are transported across said surface by entrainment in a current of the molten metal extending only to a limited depth in said bath.

3. The method defined in claim 1 wherein said particles are transported across said surface by entrainment of the particles from above without substantial movement of the molten metal of the bath.

4. The method defined in claim 1, further comprising the step of heating said bath from below by passing a hot combustion gas along an underside of the bath.

5. An apparatus for the drying of particulate minerals, comprising: receptacle means for retaining a hot liquid bath having a specific gravity in excess of that of the mineral and a free upper surface; inlet means at one side of said bath for depositing a thin layer of particles of said mineral upon said surface; discharge means at another side of said bath for removing dried mineral particles from said surface; and transport means for displacing said particles along said surface from said inlet means to said discharge means, said transport means including liquid-circulating means for generating a current of liquid of limited depth only across the surface of said bath entraining said particles from said inlet means to said outlet means.

References Cited UNITED STATES PATENTS 1,520,080 12/1924 Prichard et al. 349 1,801,263 4/1931 Burnetle 349 X 1,803,767 5/1931 OKeefe 349 KENNETH W. SPRAGUE, Primary Examiner. 

